The present invention relates to an apparatus for inspecting the state of soldered portons of a printed circuit board which carries electronic parts thereon, the measuring apparatus being characterized by the use of an optical means, and also relates to a method for the inspection.
In printed circuit boards used 1n electronic devices such as computers, the mounting of electronic parts thereon tends to become higher in packing density. At the same time, in cosideration of the radiation of heat from electronic parts, there is a tendency toward a narrower spacing between electronic parts and an increased height of parts. The height of parts to the spacing of parts ratio is 4 to 5. Further, leads of parts are becoming shorter with higher integration of electronic parts.
Under such circumstances, at the time of inspecting soldered portions between electronic parts and a printed circuit board, there arises the necessity of looking into a small soldered portion at the bottom between the parts, and such a visual inspection is no longer satisfactory. As to an automatic inspection, it is infeasible according to the prior art because of restrictions imposed on illumination and inspection.
For example, in Japanese Patent Laid-Open Publication No. 171611/83 there is disclosed an automatic inspection using an optical cutting method which recognizes a sectional shape of a soldered portion on a light scanning line to detect a defect. However, since the detection of the sectional shape is performed obliquely, the inconvenience that the portion to be inspected is hidden behind electronic parts arises with increase in packing density of the parts.
On the other hand, as a method of inspecting a soldered portion from just above, there is known a method in which the soldered portion is illuminated uniformly with a conventional vertical irradiation to detect a two-dimensional image. In this case, a strong reflected light is detected locally on part leads or the solder surface, thus resulting in that the image processing at the subsequent pickup system becomes complicated. More particularly, a correct information on shape is not obtainable because of a too broad intensity distribution of reflected light. As means for solving this problem there has been proposed a method, as disclosed in Japanese Patent Laid-Open Publication No. 232344/84, in which an excitation light is applied to a printed circuit board and fluorescence generated from the printed circuit board is detected to detect a defect of a wiring pattern on the same board.